1. Field of the Invention
The present invention relates generally to an intake air flow rate control system for an internal combustion engine of an automotive vehicle. More specifically, the present invention relates to a control strategy for controlling intake air flow rate of the internal combustion in the air flow rate control system wherein either open loop control or closed loop control is selectively carried out and a smooth transition is made between the two.
2. Description of the Prior Art
In recent years, pollution of the atmoshere by nitrogen oxides NO.sub.x, carbon monoxide CO, gaseous sulfuric acid and so on produced in the exhaust gas of automotive vehicles has become a serious social problem. In addition to this, the price of fuel, i.e. gasoline or petrol, for automotive vehicles has become higher and higher, because of the limited resources thereof. For preventing atmospheric pollution caused by exhaust gas of vehicles and for fuel economy, current automotive vehicles have been required to control engine operation so that the engine is driven in the most desirable condition even when the vehicle engine is idling.
In the intake air flow rate control system, it is preferable to carry out feedback (closed loop) control or open loop control selectively, according to engine driving condition. In feedback control, a control signal is determined corresponding to an actual engine speed measured by an engine speed sensing means such as crank shaft angle sensor and a reference engine speed determined corresponding to an engine or coolant temperature. It should be noted that, in the present specification, the word "reference engine speed" means a target engine speed theoretically determined based in engine operating parameters. Feedback control is carried out under stable engine driving conditions. Therefore, when the engine is driven unstably, feedback control should not be carried out and the intake air flow rate should be controlled by open loop control.
Therefore, it is required to switch control operation between feedback control and open loop control depending on engine conditions. In the prior art, there have been developed many kinds of switching means for switching between feedback control and open loop control. Generally, by conventional means, when control operation is switched from feedback control to open loop control, the feedback control signal is fixed at the final value immediately before switching occurs. The fixed value is maintained during carrying out of open loop control. Therefore, when control operation returns to feedback control, the first control signal determined by feedback control is the same value as the previously fixed value. If the engine driving condition is substantially changed during carrying out of open loop control, the control signal value of feedback control is quite different from the required value corresponding to the engine driving condition. This will possibly cause unstable control of the air flow rate and sometimes results in engine stall. For example, during warming-up of the engine under cold engine condition, the control signal for determining the duty cycle of a pulse signal to be applied to air flow rate control valve means in order to determine the energized period and deenergized period thereof is relatively high for the purpose of rapidly warming-up of the engine. If the engine is maintained in idling condition, the pulse duty cycle applied to the valve means is gradually reduced corresponding to increasing engine or coolant temperature. However, if the vehicle starts driving immediate after engine starting and thereby, control operation is switched from feedback control to open loop control, the pulse duty cycle determined by feedback control is fixed at a relatively high level. During driving of the vehicle, the engine or coolant is gradually warmed up. When the vehicle is stopped the engine is placed in an idling condition, the reference engine speed based on the engine or coolant temperature becomes relatively low. Thus, control mode switches from open loop control to feedback control, pulse duty cycle determined by feedback control pulse signal applied to the valve means is excessively high to temporarily increase air flow rate and thereby excessively increase engine speed. Although excessively high engine speed will be maintained for a short period of the, it will cause discomfort on the driver. Also, this will temporarily increase harmful components in the exhaust gas.
Further, after starting the engine, even though engine or coolant temperature is the same, loading of the engine varies considerably depending on the engine condition. For example, comparing engine conditions after a relatively long period of idling and immediately after driving at the same temperature, the engine load is different in each condition depending on the difference of friction between movements of internal parts and lubricant oil condition and so on. Generally, the engine load depending on internal friction or lubricant oil condition is gradually reduced and thereby the engine driving condition gradually becomes smooth. On the other hand, the reference engine speed is determined corresponding to the engine or coolant temperature. Therefore, in spite of depending on the engine load condition, the reference engine speed is determined to be the same value both after starting the engine and relatively smooth engine condition. By this, upon starting the engine and when the engine load is substantially high, engine stalling is possible.